A solid oxide fuel cell (SOFC) stack operating at high temperatures of about 550° C. to 850° C. provides an overall fuel to electric efficiency of about 40 to 50 percent, which results in a significant production of excess or waste heat. As known in the art, the term “SOFC stack” refers to a structure comprising a plurality of individual solid oxide fuel cell repeat units electrically connected in series. Each individual solid oxide fuel cell repeat unit comprises an oxygen electrode (cathode) wherein oxygen is reduced with a flow of electrons to oxide ions; a solid oxide electrolyte which transports oxide ions so produced from the cathode to a fuel electrode; the fuel electrode (anode) wherein a fuel, such as hydrogen and/or carbon monoxide, and the oxide ions are contacted to produce water and/or carbon dioxide, respectively, with concomitant production of electrons; and an external electrical circuit which collects the electrons so produced and delivers them to the cathode while also being available for useful work. In SOFC systems at least one stack is disposed within a structural housing referred to as a “stack hotbox”. Efficiently recuperating waste or excess heat from the environs around the stack as defined within the stack hotbox would enable higher fuel to electric efficiency.
The SOFC stack operates on a fuel source typically comprising hydrogen, carbon monoxide, or a mixture thereof. Hydrogen and carbon monoxide can be supplied to the stack via a steam reformer (SR) wherein a hydrocarbon fuel, such as natural gas or methane or diesel, is contacted with steam and converted in an endothermic process into a synthesis gas (syngas) comprising a mixture of hydrogen and carbon monoxide and lesser quantities of carbon dioxide and water. Alternatively, an autothermal reformer (ATR) can be employed in place of the steam reformer. In autothermal reforming, the hydrocarbon fuel is contacted with steam and an oxidant, typically air or oxygen, and converted in an exothermic process into a synthesis gas composition typically containing lower concentrations of hydrogen and carbon monoxide, as compared with syngas derived from steam reforming.
Generally, not more than about 75 percent of the synthesis gas fuel passing through the SOFC stack is utilized. An anode tail gas exiting an anode side of the stack comprises the non-utilized hydrogen and carbon monoxide as well as additional quantities of water and carbon dioxide, the additional water and carbon dioxide having been created at the anode in the fuel cell electrochemical process. The anode tail gas can be recycled to a combustor and fully combusted to carbon dioxide and water with release of exothermic heat. This exotherm from the combustor can be recuperated and utilized to drive the endothermic steam reformer.
Several advantages are achievable if the excess or waste heat radiating from the SOFC stack could be removed from the environs of the stack and utilized in a productive manner. Actively removing the waste heat from the stack would lower the temperature of the stack hotbox, which in turn would beneficially result in slower degradation and improved durability of the individual solid oxide fuel cell repeat units forming the stack. Likewise, a lower temperature of the stack hotbox would advantageously lower requirements for cathode air flow into the SOFC due to reduced cooling needs, which has the advantage of reducing pressure on seals, pumping loads, and system parasitics. Moreover, it would be advantageous to achieve a fuel utilization of greater than about 80 percent in the stack with complementary removal of increased heat. Any improvement in stack fuel utilization, however, is tied to improved thermal management of the fuel reforming.
In view of the above, a need exists in the art for an improved SOFC system in which a fuel reformer is thermally integrated with a SOFC stack in a manner that advantageously enhances thermal efficiency of the reformer while simultaneously increasing the fuel utilization of the stack, and without compromising durability and without unduly increasing system complexity. More importantly, an integrated system would result in an improvement in overall SOFC system efficiency, as defined hereinafter.